Can I change the amount of days before an event, that the rota reminders will be sent?

No you can't. All users on rotas are emailed by default 3 days before the event unless:

• the rota creator has unchecked the send rota reminders option, in which case no rota reminders will be sent.
• the user has explicitly chosen to NOT to receive rota reminders or has modified the number of days before the event that they will receive them.

  The rota creator cannot decide when a user will receive the reminder as this is the personal choice of the user.

   

  Sign-up sheet reminders are sent by default 1 day before the participants slot.